A witness quoted by Reuters news agency counted 40 bodies at the hospital, mostly those of young men believed to be students.

College provost Molima Idi Mato, speaking to Associated Press, also said the number of dead could be as high as 50, adding that security forces were still recovering the bodies and that about 1,000 students had fled the campus.

A Nigerian military source told AP that soldiers had collected 42 bodies.

The gunmen also set fire to classrooms, a military spokesman in Yobe state, Lazarus Eli, told Agence France-Presse.

The college is in the rural Gujba district.

In May, President Goodluck Jonathan ordered an operation against Boko Haram, and a state of emergency was declared for the north-east on 14 May.

Many of the Islamist militants left their bases in the north-east and violence initially fell, but revenge attacks quickly followed.

In June, Boko Haram carried out two attacks on schools in the region.

At least nine children were killed in a school on the outskirts of Maiduguri, while 13 students and teachers were killed in a school in Damaturu.

In July in the village of Mamudo in Yobe state, Islamist militants attacked a school's dormitories with guns and explosives, killing at least 42 people, mostly students.

Boko Haram regards schools as a symbol of Western culture. The group's name translates as "Western education is forbidden".

Boko Haram is led by Abubakar Shekau. The Nigerian military said in August that it might have killed him in a shoot-out.

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